This invention relates to a handpiece for dental use, and more particularly, to a contra-angle dental handpiece to be used by a dental practitioner for cutting or drilling a root canal of a tooth.
In forming a root canal of a tooth, dental cutting tools such as a dental reamer or a dental file is generally used. The dental reamer and/or dental file may be operated manually or with use of a handpiece with a drive mechanism. A dental reamer has a relatively gentle (large lead) spiral cutting edge which is rotated in a root canal for cutting a surrounding inner wall of the root canal. In contrast, a dental file has a spiral cutting edge having a lead smaller than that of the dental reamer for cutting the surrounding inner wall of the root canal. These are the basic ways in using the reamer and file, however, the reamer and file are also used by incorporating the back and forth movements as well as rotational movements at the same time.
Thus, a dental handpiece for operating such a dental reamer or file is sometimes capable of driving the reamer or file both the back and forth movements and rotational movements. This invention is directed to a contra-angle dental handpiece in which the handpiece has a shape which is bent about a right angle.
As is known in the art, a root canal is narrower toward a distal end and is also curved as a whole. Thus, it is practically impossible to visually monitor when cutting and forming the root canal until the distal end thereof, thus, involving cut-and-try like work. Further, when applying the rotational movement to the reamer or file, a continuous application of only one direction of rotation or repeated applications of both forward and reverse directions of rotation in the curved root canal tend to cause metal fatigue to the dental cutting tool (reamer or file) because of the continuous or alternate load. Furthermore, in this situation, torsional load applied to the dental cutting tool will be increased, which may break the cutting tool. To avoid the damage to the cutting tool, it is known in the art that there is a type of dental handpiece which is designed to drive the dental cutting tool both in a xe2x80x9crotary modexe2x80x9d to provide a rotational movement and a xe2x80x9creciprocating modelxe2x80x9d to provide a back and forth movement to the cutting tool.
FIG. 6 shows an example of conventional contra-angle dental handpiece proposed in Japanese Patent Laid-Open Publication No. 4-300534. FIG. 6 is a cross sectional view showing essential components of the dental handpiece. As noted above, the dental handpiece in this example is able to apply both the rotational movement (rotary mode) and the back and forth movement (reciprocating mode) to the cutting tool (reamer or file).
The dental handpiece includes a frame 3, a tool chucker 2 and a cap holder 4. The tool chucker 2 for freely mounting a cutting tool 1 (such as reamer and file) therein is attached to the frame 3 in a manner rotational about an rotational axis a. The cap holder 4 is rotatable but is not removable from the frame 3. The upper end of the cutting tool 1 has a hook which is latched by a projection of the cap holder 4 when the cap holder 4 is rotated in a small degree. Consequently, the cutting tool 1 is installed in the dental handpiece through the cap holder 4 and is moved in the direction of the rotational axis a in the reciprocating mode and is rotated about the rotational axis a in the rotary mode in response to the movement of the tool chucker 2.
The handpiece of FIG. 6 further includes an inner sleeve 5 and an outer sleeve 6 around the tool chucker 2. The outer sleeve 6 is fitted to the tool chucker 2 through, for example, a pressed-in or screwed-in process and moves both in the rotary mode and reciprocating mode with the movements of the tool chucker 2. The inner sleeve 5 is positioned between the tool chucker 2 and the outer sleeve 6 and independently rotates and moves back and forth.
The frame 3 of the handpiece is bent at about right angle where a crank shaft 7 is rotatably provided in a direction perpendicular to the cutting tool 1 (rotation axis a). The crank shaft 7 is driven by a drive mechanism such as a motor (not shown). The crank shaft 7 has, at its end, an eccentric shaft 8 and an eccentric ball 9 at the end of the eccentric shaft 8.
The outer sleeve 6 has a guide groove 6a in which the eccentric shaft 8 of the crank shaft 7 is engaged. When the crank shaft 7 rotates, the eccentric shaft 8 rotates accordingly, thereby driving the outer sleeve 6 in the direction of the rotation axis a, although there is no rotation of the outer sleeve 6. With the back and forth (reciprocating) movement of the outer sleeve 6, the tool chucker 2 moves back and forth accordingly.
The surrounding wall of the inner sleeve 5 has a long hole 5a in the direction of the rotation axis a which is engaged with the ball 9 formed on the end of the crank shaft 7. Thus, when the crank shaft 7 rotates, the ball 9 rotates accordingly, thereby rotating the inner sleeve 5 in the forward and reverse direction about the rotation axis a (rotary mode). However, the inner wall 5 is not driven by the ball 9 back and forth in the direction of the rotation axis a (reciprocating mode).
In the foregoing configuration of the handpiece of FIG. 6, by the rotation of the crank shaft 7, the eccentric shaft 8 rotates accordingly, thereby driving the tool chucker 2 in the reciprocating mode and the rotary mode. Namely, in the reciprocating mode, the handpiece drives the tool chucker in the direction of the rotation axis a. In the rotary mode, the handpiece rotates the tool chucker 2 about the rotation axis a. In other words, the cutting tool 1 (reamer or file) moves back and forth in the direction of the rotation axis a while rotates about the rotation axis a in the forward and reverse direction at the same time.
The foregoing example of the conventional handpiece further includes a switch means so as to switch between the operation involving only the reciprocating mode (back and forth movement in the direction of axis a) driven by the eccentric shaft 8 and the other operation involving both the reciprocating mode and the rotary mode (forward and reverse rotation about the axis a) driven by the eccentric shaft 8 and the ball 9. Such switching mechanism is unrelated to the point of the present invention, and thus, no further description is given here.
As in the foregoing, with use of the handpiece of FIG. 6, a root canal of a tooth can be effectively formed by the reciprocating mode and the rotary mode of the cutting tool 1. As noted above, the reciprocating mode involves the back and forth movement of the cutting tool 1 in the axial direction of the axis a and the rotary mode involves the forward and reverse rotation of the cutting tool 1, which is typically less than one rotation, i.e., 360xc2x0.
In the process of forming a root canal of a tooth, it is important to carefully operate the handpiece so that the cutting tool such as a reamer or file is driven in the reciprocating movement as well as the rotary movement. As shown in FIG. 7, in the cutting process of a curved root canal 10, the cutting tool 1 is sent to a root opening 10a at the end of the root canal 10 and is rotated to cut the tissue in the area shown by the dotted line, thereby forming a seat at the root opening 10a. Under this situation, if the cutting tool 1 is driven by the continuous rotation in one direction at a high speed, or high speed repetition of the forward and reverse rotation, metal fatigue may be caused, which may damage the cutting tool 1.
Further, in forming the root canal of the tooth, the debris generated by cutting the inner wall of the root canal must be effectively carried away to the outside. This is because the debris is full of germs causing inflammation and suppuration after the dental treatment if the debris is not sufficiently removed.
In the root canal treatment based on the manual operation of the cutting tool, it is desirable to repeat the process of rotating the cutting tool in the forward direction at the position most advanced and retreating the cutting tool therefrom. It is very important to rotate the cutting tool in the forward direction by less than one rotation, such as by a xc2xc or ⅕ rotation at the intended cutting position (most advanced position) in the root canal.
This is because, first, such a repetition of the process can minimize the stress brought to the cutting tool, thereby avoiding the damage and extending the life time of the cutting tool. An example of rotation angle is sufficiently less than an angle in which the cutting tool has a problem in digging into the root canal wall, such as less than 180xc2x0, preferably in the range about 20xc2x0-45xc2x0.
Second, the debris of the root canal produced by the rotation of the cutting tool having a spiral cutting edge has to be removed toward the outside by the forward rotation of the spiral cutting edge.
Third, as shown in FIG. 7, since the root canal is curved, the cutting tool 1 is curved along the root canal. Under this situation, the end of the cutting tool 1 contacts the outer curve of the root canal 10. Thus, if the cutting tool 1 is rotated when retracted (backward movement in the reciprocating mode), an intermediate portion of the root canal may be cut, forming a step 10b as shown in FIG. 8 or cutting the root canal excessively.
Therefore, a contra-angle handpiece is desired to have a capability that it can rotate the cutting tool in the forward direction at the intended cutting location, i.e., most advanced position, in the root canal.
As noted above, the conventional contra-angle handpiece of FIG. 6 can drive the cutting tool in the back and forth in the direction of axis a as well as forward and reverse rotation about the axis a. However, the handpiece of FIG. 6 is not specifically designed to apply the forward rotation to the cutting tool when advancing the cutting tool in the root canal or where at the most advanced position (drilling position) in the root canal.
In the example of handpiece of FIG. 6, it is possible to achieve the forward rotation of the cutting tool while advancing forward in the root canal if positions of the eccentric shaft 8 and the ball 9 are so adjusted. However, in such a case, the cutting tool rotates in the reverse direction as well when retracted back in the root canal. This may cause excessive fatigue on the cutting tool or sending back the debris to the inside of the root canal. Moreover, since it is not designed with specific intention to rotate the cutting tool at the most advanced position in the root canal, an excessive cut, such as the step of FIG. 8, may be caused in the root canal.
This invention has been made to solve the problems involved in the dental handpiece in the conventional technology.
It is, therefore, an object of the present invention to provide a contra-angle dental handpiece which is capable of rotating the cutting tool in the forward direction when the cutting tool is in the most advanced position (cutting position).
It is another object of the present invention to provide a contra-angle dental handpiece which is capable of preventing the reverse rotation of the cutting tool when the cutting tool is retracted from the most advanced position in the root canal, thereby promoting the removal of debris generated by cutting the tissue of the root canal.
In the present invention, a contra-angle dental handpiece includes a rotation shaft for rotating a dental cutting tool about a rotation axis, a drive shaft for rotating the rotation shaft, and a drive mechanism for driving the dental cutting tool in a backward and forward direction of the rotation axis and for at least rotating the dental cutting tool in the forward direction about the rotation axis when the cutting tool is at around a most advanced position in a root canal of a tooth. The contra-angle dental handpiece further includes a tool chucker for attaching the cutting tool to the dental handpiece.
The drive mechanism in the contra-angle dental handpiece is comprised of a receptacle means provided directly or indirectly on the cutting tool to produce the rotation of the cutting tool, and a coupling means provided on the drive shaft to temporarily couple with the receptacle means during the rotation of the drive shaft to move the receptacle means in the backward and forward direction in the direction of said rotation axis. The coupling means rotates the receptacle means in the forward rotational direction when the receptacle means is moving in the forward direction of the rotation axis and releases the coupling to the receptacle means when the receptacle means is moving in the backward direction of the rotation axis.
More particularly, the drive mechanism in the contra angle dental handpiece is comprised of first and second circular plates provided on the tool chucker in a parallel fashion with a predetermined distance therebetween, and an engagement projection connected to an end of the drive shaft and is projected in a space between the first and second circular plates. The second circular plate has a plurality of grooves one of which receives said engagement projection therein when the engagement projection rotates by the rotation of the drive shaft. The engagement projection drives the first circular plate to move the tool chucker backward in the direction of the rotation axis and drives the second circular plate to move the tool chucker forward in the direction of the rotation axis. The engagement projection rotates the tool chucker only in the forward rotational direction when the tool chucker is moving in the forward direction of the rotation axis.
The drive shaft may be comprised of first and second drive shafts each being operated independently from each other wherein the first drive shaft drives the forward and backward movement of the tool chucker and the second drive shaft drives the rotation of the tool chucker.
The contra-angle dental handpiece may further comprises a first spring means for pressing the first circular plate in the forward direction of the rotation axis and a second spring for pressing the second circular plate in the backward direction of the rotation axis.
In a further embodiment, the receptacle means is a first bevel gear connected directly or indirectly to the cutting tool and the coupling means is a second bevel gear connected to the drive shaft. The first bevel gear is larger than the second bevel gear in diameter.
According to the present invention, the contra-angle handpiece can drive the dental cutting tool such as a dental reamer or a dental file both in the reciprocating mode and the rotary mode, and can apply at least the forward rotation to the cutting tool when the cutting tool is in the most advanced position in the root canal. In the present invention, it is possible to effectively prevent the cutting tool from being damaged and extend the life time of the cutting tool. The dental handpiece of the present invention can promote removal of the debris from the root canal and can establish a clean root canal without involving excessive cutting in the inner wall of the root canal.